1. Field of the Invention
The present invention relates to an apparatus and a method for calculating a feature quantity in connection with microvibration.
2. Description of the Background Art
In recent years, the development of technology of a system that uses outputs from a Doppler sensor to detect a heart rate has been promoted. A Doppler sensor radiates a radiation wave at a predetermined frequency to an object to be sensed and senses part of the radiation wave reflected by the object to output an output signal representative of a difference between the frequency of the radiation wave and a frequency of the reflected part. Accordingly, the use of such a Doppler sensor makes it possible to detect the heart rate of a test subject without touching the test subject. Heart rate detecting solutions using a Doppler sensor are disclosed by, for example, Japanese Patent Laid-Open Publication Nos. 2006-55504 and 2010-142456.
In the solution disclosed in Japanese '504 Publication, two sensors are used, one of which is a radio wave type of Doppler sensor that radiates a radio wave without contacting the surface of a human body to receive a reflected wave from the human body surface, and the other of which is a low-frequency sensor which is disposed near the human body surface to output an amplitude component associated with a variation in blood flow inside the human body so as to discriminate changes in heartbeat of a test person from his or her movement to measure the heart rate.
Japanese '456 Publication discloses a solution in which a Doppler sensor is embedded into a driver's seat of a vehicle, and the heart rate of a driver is estimated from outputs from the Doppler sensor. Further, in the solution, an RRI (R-R Interval), which is an interval between adjacent two peak positions (R) on heartbeat waves, is calculated with positions of the heartbeat of a driver identified so as to determine when the driver loses his or her powers of concentration or the like. In this solution, positions at which heartbeat possibly exists in time-serial data output from the Doppler sensor are predicted by means of past positional data, and information on amplitudes included in the prediction result is mainly used to identify positions of heartbeat. Further, outputs from the Doppler sensor are filtered by a band-pass filter to extract only components closely associated with the heartbeat for further processing.
However, in the above-described method disclosed in Japanese '504 Publication, the one sensor has to be disposed near a human body, thus bringing about a problematic restriction on the environment in which heartbeat is measured. In the above-described method disclosed in Japanese '456 Publication, positions of heartbeat are identified on the basis of output signals per se from the Doppler sensor or amplitudes of frequency components which are possibly associated with heartbeat, thus bringing about a difficulty in detecting heartbeat unless the sensor is very closely located to a human body.